If a QB is on the practice squad, then that QB can be taken, at any time, by another team with QB needs when signed to the active roster (injuries were implicit in this thread, which means many teams have QB needs and search the best available). There is no farm team system in the NFL. You do not send an NFL player to the minors under contract, and elevate him exclusively to your team when he's ready.
The practice squad players are emergency relief, hopefully with just enough talent to make them attractive to the team with that practice squad and no others. Educating a practice squad QB, ie. player development, on your offensive system creates a major liability when that player is signed by another team. That person is trained to command the offense, and therefore leaves with a good chunk of the playbook. Why would you want to do that? Or invest time and energy into developing a QB, who becomes good enough and joins another NFL team on the active roster?
This is not the Durham Bulls. This is not the PawSox. If a player is good enough, then he is on the active roster or another team's active roster. Marginal misses become practice squad fodder, and it is hoped no other team has an interest. Mallett is the development project now. There is a 53-man roster, so precisely how many QBs need to be developed at one time? What are the odds a third tier QB will be called to action, and will not simply occupy a role a more diverse player could use more productively?
If a player on the PS is good, then he gets snatched up. Why not snap up some JAG veteran free agent or other team's practice squad QB when the need arises in lieu of that scenario if the practice squad QB does not appeal to 31 other NFL teams? Is that PS player likely to make your team more competitive, inferior abilities and all, if he has spent time on the practice squad? Will a more talented free agent with less system knowledge represent an inferior choice as an emergency option?